For many years, liquid-liquid extraction (LLE) using sulfolane or polyalkylene glycol as the extractive solvent has been the most important commercial process for purifying the full-range (C6-C8) of aromatic hydrocarbons from petroleum streams, including reformate, pyrolysis gasoline, coke oven oil, and coal tar. Extractive distillation (ED) with N-methylpyrrolidone as the extractive solvent has also been extensively applied for benzene recovery from coal tar and coke oven oil. Recently, ED using sulfolane solvent became commercially viable for benzene and toluene recovery from reformate or pyrolysis gasoline after C8+ fractions are removed from the feedstock. The extractive solvent in both ED and LLE processes for aromatics recovery is internally circulated indefinitely in the process system in a closed loop.
Typically, the ED or LLE feedstock is fed to a prefractionation column for removing the heavy portion and leaving only the desirable portion to be fed to the ED column or LLE column. Even for well-designed prefractionation columns, under reasonable operating conditions, some measurable amount of heavy hydrocarbons will slip into the feed stream to the ED or LLE process. And under poorly operated or malfunctioned prefractionation columns, the amount of heavy hydrocarbons in the feed stream increases significantly. Subsequently, the concentration of heavy hydrocarbons as well as polymeric materials, which are generated by the interactions among the heavy hydrocarbons, decomposed solvent, solvent additives and species from equipment corrosion, can increase quickly, thereby deteriorating solvent performance. In severe cases, it could render the process inoperable.
U.S. Pat. No. 4,820,849 to Diaz describes a process for reducing the level of corrosive impurities in sulfolane solvent originating from a process for the extraction of aromatic hydrocarbons from petroleum, having a pH of at least 8.5. The process combines a sulfolane-soluble polyprotic acidic substance with the sulfolane to form a solid phase containing at least a portion of the corrosive impurities and separates the sulfolane from the solid phase. The polyprotic acidic substance is sulfuric acid or phosphoric acid. The method is tedious and requires acid addition and solids handling, and deals with only the corrosive impurities in the solvent. It is not applicable for the removal of heavy hydrocarbons or polymeric materials. A regeneration and/or purification method disclosed in U.S. Pat. No. 5,053,137 to Lal uses a pair of columns arranged in series, with the first column containing cation exchanger resin and the second containing anion exchanger resin, to remove ionic and polar impurities from the solvent (sulfolane).
To remove heavy hydrocarbons and polymeric materials and polar impurities derived from oxidized solvent, a method applied extensively in commercial LLE or ED processes employs a thermal solvent regenerator, where a small slip stream, of lean solvent (approximately 1-2% of total lean solvent stream) is heated with or without stripping steam in order to recover the regenerated solvent or any heavy, components having boiling points lower that of the solvent. The heavy polymeric materials, having boiling point higher than that of the solvent, are removed from the bottom of the solvent regenerator as sludge. The basic concepts of this thermal solvent regeneration scheme are described in U.S. Pat. Nos. 4,046,676 and 4,048,062 both to Asselin in relationship to a LLE process for aromatics recovery where a portion of lean solvent from the bottom of the solvent recovery column (SRC) is diverted into a solvent regeneration zone. A vaporous stripping medium (steam) is introduced into the solvent regeneration zone separately, recovered with regenerated solvent and introduced into the SRC as at least a portion of the stripping steam. When applied to LLE processes using sulfolane/water, or polyalkylene glycol/water as the extractive solvent, thermal solvent regeneration has been commercially successful in keeping the heavy hydrocarbons and polymeric materials at a tolerable level in the lean solvent. This is because a significant amount of heavy hydrocarbons (C9-C12+) in the feedstock is rejected by the solvent phase in the LLE column and is removed with the raffinate phase as a part of the non-aromatic product. For the same type of molecules, the higher the boiling point, the lower the polarity. Among the heavy hydrocarbons, only C9+ aromatic compounds are likely to be extracted by the solvent, which can be almost entirely stripped from the solvent in the SRC under normal operating conditions.
In a normal ED process for aromatics recovery, however, all of the heavy hydrocarbons tend to remain in the rich solvent at the bottom of extractive distillation column (EDC) due to their high boiling points. Even for the narrow boiling-range (C6-C7) feedstock for benzene and toluene recovery, there can be 3-5% heavy hydrocarbons trapped in the solvent, in spite of increase in the severity of the SRC (higher temperature and vacuum level, and more stripping steam) to drive additional heavies from the lean solvent. For the full boiling-range (C6-C8) feedstock for benzene, toluene and xylene recovery, however, the boiling points of most heavy hydrocarbons are too high to be stripped from the solvent in the SRC and consequently they accumulate in the solvent since the solvent is circulated between the EDC and the SRC indefinitely in a closed loop.
The above described solvent regeneration schemes are not suitable for the ED processes since they were designed specifically for LLE processes for removing relatively minor amounts of polymeric materials formed possibly from the reactions between the oxidized or decomposed solvent components and trace of the heavy hydrocarbons in the solvent. Indeed, when these solvent regeneration schemes were implemented with ED processes, heavy hydrocarbons tend to accumulate and polymerize in the closed solvent loop. This buildup continues until the polymerized materials achieve boiling points higher than that of sulfolane (>287° C.), whereupon they exit the closed loop through the bottom of the solvent regenerator. It is a potentially disastrous situation since excessive polymeric materials in the solvent not only significantly changes the solvent properties (selectivity and solvency), but also plugs process equipment, such as, pumps, valves, column internals, lines, etc., to render the ED process inoperable.
To take advantage of the fact that most extractive solvents for ED and LLE are water soluble, U.S. Pat. No. 7,666,299 to Wu adopts a different approach for removing heavy hydrocarbons from the extractive solvent whereby lean solvent is introduced into a low temperature, energy saving and easy-to-operate solvent washing zone and contacted with a stream of process water, which is circulated in closed loop. Solvent is dissolved into the water phase, while heavy hydrocarbons are rejected by water and accumulated into the hydrocarbon phase. At a minimum, the solvent washing zone serves as a decanter to remove and separate the minor heavy hydrocarbon phase from the bulk water phase. The decanted hydrocarbon phase accumulates and is withdrawn from top of the decanter periodically. In one configuration, the washing water contacts lean solvent in a counter-current fashion to extract the solvent into the water phase and to reject the heavy hydrocarbons and other water-insoluble into the oil phase. The water phase containing essentially the purified solvent is withdrawn continuously from the lower portion of the contactor. A minor hydrocarbon phase accumulates at the top of the contactor and is removed periodically from the contactor under level control. Any solids precipitation formed in the solvent washing zone is removed from the bottom of the contactor. Since this method requires a significant amount of water, it may be difficult to balance and distribute the process water in the closed system.